Review rapid home test kits with caution
Rapid at-home diagnostic testing for the pandemic virus should be as easy as using the morning toaster. Unfortunately, rapid tests are in short supply just when they are needed most. Aside from availability, questions are bubbling up about whether rapid diagnostic tests are working. Has the omicron variant, with its many mutations and extreme transmissibility, compromised their effectiveness?
No. Rapid tests can still detect omicron, but with limits. The athome, rapid self-test, useful in series (two or more times) within the first seven days after infection, checks for a protein from the virus that causes covid-19, and delivers a result in about 15 minutes. Epidemiologist Katelyn Jetelina of the University of Texas points out that antigen tests “target a fairly stable part of the virus called the nucleocapsid,” which has fewer mutations than the spike protein. “This is fantastic news because it means that we still have a tool to help calm the Omicron storm.”
But there are signs of difficulty. The Food and Drug Administration said Dec. 28, “Early data suggests that antigen tests do detect the omicron variant but may have reduced sensitivity.” The FDA’s statement was based on laboratory tests, not clinical data. Experts have suggested one reason for the reduced sensitivity is that antigen tests are based on a nasal swab, but the omicron variant tends to replicate in the upper airways, not so much deep in the lungs as earlier variants. So saliva might be a better source of sampling.
Another doubt about antigen tests came when the director of the Centers for Disease Control and Prevention, Rochelle Walensky, ruled out requiring a negative antigen test to exit a five-day quarantine period. She said it wasn’t clear the tests would be effective at measuring a person’s contagiousness after five days. The government has said rapid tests are not designed to detect whether a person is infectious. (Although logically, if an antigen test is positive, the person might well be contagious.) A fact sheet packaged with the Abbott BinaxNOW antigen test says that after five days, the antigen test “may be more likely to be negative” than the more accurate saliva-based laboratory test known as PCR, or polymerase chain reaction, which detects genetic material from the virus.
Another question mark was raised Jan. 5 in a preliminary study of a small sample of people in workplace settings. It found antigen tests failed to pick up omicron in the first several days of infection. This is worrisome, suggesting a window in which people could be contagious when an antigen test is negative.
Rapid antigen tests are useful but not perfect. A positive test likely indicates high viral loads and transmissibility. But very early in an infection, and later on, an antigen test might not detect lower viral loads. A negative result should be viewed with caution, especially if there are symptoms or an exposure is suspected. When in doubt, all the other measures for managing the virus — masking, isolation and more testing — should be employed, too.